Field of the Invention
In one of its aspects, the present invention relates to a process for treatment of a fluid comprising an oxidizable contaminant. In another of its aspects, the present invention relates to a process for treatment of a petroleum refinery wastewater comprising an oxidizable contaminant and a sulfide.
Description of the Prior Art
It is generally known that contaminants, such as organic pollutants, present in industrial wastewater and contaminated groundwater can be oxidized and destroyed by hydroxyl radicals (.OH).
Generally, this hydroxyl radical can be produced by a variety conventional processes, including:                UV irradiation of hydrogen peroxide (Baxendale, J. H. and Wilson, J. A. (1957). The photolysis of hydrogen peroxide at high light intensities Trans. Faraday Soc. 53, 344-356).        The so-called Fenton reaction, in which ferrous (Fe(II)) (Walling, C. (1970). Fenton's reagent revisited, Acc. Chem. Res. 8, 125-131) or ferric (Fe(III)) (Pignatello, J. J. (1992). Dark and photoassisted Fe3+-catalyzed degradation of chlorophenoxy herbicides by hydrogen peroxide, Environ. Sci. Technol. 26, 944-951) ion react with hydrogen peroxide to produce hydroxyl radicals.        The photo-assisted Fenton process, in which a mixture of ferric ion (Sun, Y., and Pignatello, J. J. (1992), Chemical treatment of pesticides wastes. Evaluation of Fe(III) chelates for catalytic hydrogen peroxide oxidation of 2,4-D at circumeutral pH, J. Agric. Feed Chem. 40, 322-327) or an iron(III)-chelate (Zepp, R. G.; Faust, B. C., and Hoigne, J. (1992), Hydroxyl Radical Formation in Aqueous Reactions (pH 3-8) of Iron(II) with Hydrogen Peroxide: The Photo-Fenton Reaction, Environ. Sci. Technol. 26, 313-319) and hydrogen peroxide is irradiated with UV and/or UV/vis. light, etc.        
The major drawbacks of these conventional approaches include: (i) for practical purposes, UV irradiation of hydrogen peroxide is limited to water with relatively high UV transmission and low level of contamination, and (i) for practical purposes, the Fenton and photo-assisted Fenton processes require a relatively acidic environment (e.g., pH 2-4) due to the iron solubility issue.
Sun and Pignatello (ibid) have shown that Fe(III) forms soluble complexes with a variety of organic and inorganic compounds at pH 6.0 in aqueous solution and that some of iron(III)-chelates can act as the Fenton reagent and can be used for the oxidation of 2,4-dichlorophenoxy acetic acid. The most active ligands were rodizonic acid, gallic acid, hexaketocyclohexane, picolinic acid, N-(hydroythyl)ethylenediaminetriacetic acid and tetrahydroxy-1,4-quinone hydrate. All but one of these ligands are expensive and/or may not be highly stable or readily available.
Walling et al. (Walling, C., Kurtz, M., and Schugar, H. J. (1970). The iron(III)-ethylenediaminetetracaetic acid peroxide system, Inorg. Chem. 9, 931-937), Francis et al. (Francis, K. C., Cummins, D., and Oakes, J. (1985). Kinetics and structural investigations of [FeIII(edta)]-[edta-ethylenediamine-tetra-actate(4-)] catalyzed decomposition of hydrogen peroxide, J. Chem. Soc. Dalton Trans., 493-501), Rahhal et al. (Rahal, S. and Richter, H. W. (1988). Reduction of hydrogen peroxide by the ferrous iron chelate of diethylenetetradiamine-N,N,N′,N″,N″-pentaacetate, J. Am. Chem. Soc. 110, 3126-3133, Sun and Pignatello (ibid), U.S. Pat. No. 6,960,330 (Cox), Dao et al. (Dao, Y. H., and De Laat, J. (2011). Hydroxyl radical involvement in the decomposition of hydrogen peroxide by ferrous and ferric-nitrilotriacetate complexes at neutral pH, Wat. Res. 45, 3309-3311) and others (for a review see Pignatello, J. J., Oliverous, E., and MacKay, A. (2006). Advanced oxidation processes for organic contaminant destruction based on the Fenton Reaction and related chemistry, Critical Rev Environ. Sci. Technol., 36, 1-84.) have shown that iron(III)-chelates (also referred to herein as FeIII-L) also act as the Fenton reagent and are able to decompose hydrogen peroxide in circumneutral pH conditions (pH 6.5-7.5) according to following reactions:FeIII-L+H2O2↔FeIII-L(H2O2)→FeII-L+HO2./O2.+H+  (1)FeIII-L+HO2./O2.→FeII-L+O2  (2)
FeII-L generated in the above reaction reacts with hydrogen peroxide and generates hydroxyl radical (.OH) according to the following reaction:FeII-L+H2O2→FeIII-L+OH−+.OH  (3)
Hydroxyl radicals generated in reaction (3) can react with, and oxidize, organic pollutants in water and soil (Dao and De Laat (ibid), U.S. Pat. No. 6,960,330 (Cox, Jr.) and U.S. Pat. No. 6,160,194 (Pignatello)). The chelating agents that can be used include aminopolycarboxylates and their phosphorous-containing analogues, for example, ethylendiaminetetraacetic acid (EDTA), nitrilotriacetate (NTA), methyglicenediacetic acid (MGDA), phosponomethyliminodiacetic acid (PMIDA) and the like.
Unfortunately, the rate of generation of OH-radical from iron(III)-chelate catalyzed decomposition of hydrogen peroxide is very slow. For example, the rate constant of reaction between FeIII-nitrilotriacetate, which is a relatively active iron(III)-chelate catalyst, and hydrogen peroxide varies from 16-27 M−1s−1 (De Laat, J., Dao, Y. H., El Najjar, N. H., and Daou, C. (2011). Effect of some parameters on the rate of the catalysed decomposition of hydrogen peroxide by iron(III)-nitrilotriacetate in water, Wat. Res. 45, 5654-5664), depending on solution pH. As a result, treatment of contaminated water employing iron(III)-chelates as the Fenton catalyst is time consuming and impractical for most industrial applications, in general, and for the oxidation of benzene and other aromatic hydrocarbons in oil and gas refinery (OGR) wastewater, in particular.
Thus, despite the advances to date, there is an ongoing need for an improved approach for treatment of contaminated water employing iron(III)-chelates as the Fenton catalyst for industrial applications, in general, and for the oxidation of benzene and other aromatic hydrocarbons in oil and gas refinery (OGR) wastewater, in particular.